In the aging hematopoietic system, lymphoid progenitors exhibit severe growth defects, while myeloid progenitors remain relatively unperturbed. Furthermore, old B cell progenitors are refractory to transformation. The central hypothesis of this application is that these dichotomous effects of aging on blood cell development and patterns of leukemogenesis are due to the preferential expression of the p16Ink4a and Arf proteins, which are products of the Cdkn2a locus, in the lymphoid lineage. Aim 1 will define when and at which stages of B lymphopoiesis expression of p16Ink4a and Arf occurs and determine how their expression affects the growth and survival of B cell progenitors. Aim 2 will use both loss and gain of function approaches to define the relative contribution of p16Ink4a and Arf to the age-related declines in B cell development and determine whether down-regulating their expression can reverse that process. Recent studies from our laboratory have shown that the malignant capacity of hematopoietic progenitors evolves in a manner that mirrors aging. Aim 3 will again use loss and gain of function approaches to determine how expression of p16Ink4a and/or Arf in aging B lineage cells underlies this pattern of leukemogenesis. An important goal of these studies is to develop proof of concept data showing that induction of senescence in transformed lymphoid progenitors will be of therapeutic value. In addition to providing insights into the effects of aging on the B cell lineage, the results of these studies will provide `proof of concept'data that manipulating the expression of p16Ink4a and Arf will be of value in rejuvenating B lymphopoiesis in the aged and treating leukemia. PUBLIC HEALTH RELEVANCE: Reduced B cell production that accompanies aging is thought to be one reason for the decline in immunity in the elderly. If this process could be better understood, it could lead to the development of therapies designed to rejuvenate B cell production. In addition, the experiments in this proposal are relevant to understanding patterns of leukemogenesis and may suggest novel approaches for treating lymphoid leukemias.